1. Field of the Invention
The present invention is related to a DC motor control method for avoiding the reverse current, and more particularly, to a DC motor control method which can prevent the reverse current induced by Back Electromotive Force (BEMF) inherent in a DC motor.
2. Description of the Prior Art
A DC (direct current) motor has very wide application in our everyday life. For example, fans installed on a CPU, or on a casing of a personal computer or inside a projector, and these are the cases which the DC motor are used for dissipating heat; other cases like stirrers, toys or robots are also using DC motors for various purposes. Generally speaking, the working principle of a DC brushless motor is to conduct a current through a stator coil of the DC motor to generate a magnetic field, and then the magnetic field is interacted with the magnetic field of a rotor (armature) to generate mechanical torque. Meanwhile, the DC motor can generate a Back Electromotive Force whose polarity is opposite to the polarity of the applied voltage. The Back Electromotive Force is naturally generated when the DC motor is running and is related to constitutive materials and a rotating speed of the motor, and can reduce the magnitude of the motor current.
Please refer to FIG. 1, which illustrates a schematic diagram of a DC motor control circuit 10 of the prior art. The DC motor control circuit 10 is mainly utilized to control a DC motor MOTOR1, and is composed of a power supply device 100, a motor controller 102, power switches PSW1˜PSW4, a PWM (pulse width modulation) signal generator 104 and a Hall sensor 106. Furthermore, the DC motor control circuit 10 includes a capacitor CVM1 to stabilize voltage, and a diode D1, which is used for protecting the power supply device 100. Inside FIG. 1, the DC motor MOTOR1 is represented by an inductor LEQ1, a voltage source representing a Back Electromotive Force VBEMF1, and a resistor REQ1. In many applications of DC motors, a pulse width modulation (PWM) technique is utilized to control the magnitude of the motor current, such that electric energy can be saved and the rotating speed can be set under control. PWM technique can also regulate energy delivered to the load (which is the DC motor MOTOR1 in this case) from the power supply device 100 by changing the length of the turn-on time within a machine cycle, and the ratio of the turn-on time relative to length of a period is often called the duty cycle. When the duty cycle approaches 1.0, it represents the power supply device 100 is delivering the energy to the load almost in full capability; on the other hand, if the duty cycle approaches 0, it represents the power supply device 100 is only sending very limited power to the load. Besides that, the control of the DC motor needs the Hall sensor 106 to provide a sensing signal, which can indicate the current positions of the armatures and the rotating speed of the DC motor MOTOR1; therefore the DC motor MOTOR1 may include one or more Hall sensors to help controlling the switching operations of the power switches PSW1˜PSW4, such that the DC motor MOTOR1 can be operated more conveniently and precisely.
Please keep referring to FIG. 1, the motor controller 102 receives signals produced by the PWM signal generator 104 and the Hall sensor 106, and generates four timing control signals to control the switching operations of the power switches PSW1˜PSW4, wherein the power switches PSW1˜PSW4 constitute an H-bridge. According to detecting results of the Hall sensor 106, which are mainly related to positions of the armature, the motor controller 102 operates alternatively in two motor driving states (the first motor driving state and the second motor driving state) to supply electrical energy to the motor MOTOR1. In the first motor driving state, the motor controller 102 turns on the upper gate bridge PSW1 and the lower gate bridge PSW2, such that current can be conducted from the power supply device 100, through the upper gate bridge PSW1 and then through the motor MOTOR1, and finally directed to the ground via the lower gate bridge PSW2; electrical energy is delivered to the motor MOTOR1 in this way while in the first motor driving state. In the second motor driving state, the motor controller 102 turns on the upper gate bridge PSW3 and the lower gate bridge PSW4, such that current can be conducted from the power supply device 100, through the upper gate bridge PSW3, then the motor MOTOR1, and finally directed to the ground via the lower gate bridge PSW4; electrical energy is delivered to the motor MOTOR1 while in the second motor state. By operating between the first motor control state and the second motor control state in turn, the motor can be rotated smoothly. Besides that, the motor controller 102 can regulate the amount of electrical energy delivered to the motor MOTOR1 by changing the duty cycle of the PWM signal, such that the motor speed can be properly controlled and the circuit can be of higher energy efficiency.
Please refer to FIGS. 2A and 2B. FIG. 2A illustrates operating states of the power switches PSW1˜PSW4 and timing diagrams of the related circuit nodes after the DC motor control circuit 10 starts to deliver energy to the motor MOTOR1, and FIG. 2B illustrates a schematic diagram showing the direction of the current when the DC motor control circuit 10 starts to deliver energy to the motor MOTOR1. For clarity, FIG. 2A only includes the timing diagrams of the terminals OUTA1, OUTB1 of the motor, the endpoint VM1, the corresponding Back Electromotive Force VBEMF1 of the motor, and the motor current IL_1, to show the operating conditions in the first motor driving state. Inside, the voltage levels of the terminals OUTA1 and OUTB1 can be expressed as the following equation:VOUTA1=VVM1−IL—1·RDS-ON,andVOUTB1=IL—1·RDS-ON.
According to the equations above, VOUTA1 and VOUTB1 respectively represent the voltage levels at the terminals OUTA1 and OUTB1; VVM1 represents the voltage value at the endpoint VM1, and RDS-ON represents the conducting resistance of the related power switch. Also, in FIG. 2A, since the power switches PSW1 and PSW2 have been turned on by the motor controller 102, the power supply device can deliver energy to the motor, and the motor current IL_1 can increase with a positive slope. The value of the slope can be expressed as the following equation:
      m    UP    =                    V                  VM          ⁢                                          ⁢          1                    -              (                  REQ          ⁢                                          ⁢                      1            ·            IL_                    ⁢          1                )            -              VBEMF        ⁢                                  ⁢        1                    LEQ      ⁢                          ⁢      1      where mUP represents the rising slope of the motor current IL_1, and the resistor REQ1 represents the internal resistance of the motor MOTOR1. Noteworthily, since the conducting resistance of the power switch imposes comparatively less influence on the value of slope mUP, it is ignored in the equations above for simplicity.
Please refer to FIG. 2C, which covers the operating conditions for more PWM cycles than FIG. 2A. Still operated in the first motor driving state as shown in FIG. 2A, and the PWM scheme is applied to regulate the magnitude of the motor current, FIG. 2C illustrates a schematic diagram of the states of the power switches PSW1˜PSW4 and timing diagrams of the related circuit nodes for the first several PWM periods after the DC motor control circuit 10 starts to deliver energy to the motor MOTOR1. Inside FIG. 2C, it shows that when the power switch PSW1 is turned off, the power supply unit stops providing power to the motor MOTOR1, and the motor current IL_1 starts to decrease with a negative slope, which can be expressed as the following equation:
            m      DOWN        =                            VBEMF          ⁢                                          ⁢          1                -                  (                      REQ            ⁢                                                  ⁢                          1              ·              IL_                        ⁢            1                    )                            LEQ        ⁢                                  ⁢        1              ,where mDOWN represents the falling slope of the motor current IL_1. Besides that, please refer to FIG. 2D, which illustrates a schematic diagram demonstrating the direction of the current in the DC motor control circuit 10 while the power switch PSW1 is turned off. Similarly, since the conducting resistance of the power switch imposes comparatively less influence on the value of slope mUP, it is ignored in the equation above for simplicity.
While in normal operation, the motor controller 102 can use the PWM technique to control the motor speed and save the energy. Within a PWM period (cycle), as the PWM signal is in the high voltage level (PWM=High), the motor controller 102 turns on an upper gate switch such that the electrical energy is transferred to the motor MOTOR1, and the motor current IL_1 is increased according to the rising slope as stated above; on the other hand, when the PWM signal is in the low voltage level (PWM=Low), the motor controller 102 turns off the upper gate switch, and stops delivering the electrical energy to the motor MOTOR1, and it shows that the motor current IL_1 will decrease according to the falling slope (as stated above) till the end of the current PWM cycle.
For some applications, the motor speed needs to be decreased largely within a very short period of time, and this can be realized by regulating the duty cycle of the PWM signal. Please refer to FIG. 3A, which illustrates a schematic diagram of states of the power switches PSW1˜PSW4 and timing diagrams of the related circuit nodes in the DC motor control circuit 10 while the duty cycle of the PWM signal exhibits a sudden change. In this case, if the duty cycle suddenly drops from 90% to 10%, at that moment (within a short time), the motor will keep rotating in a high speed owing to the rotational inertia of the rotor. Also, since the motor's Back Electromotive Force VBEMF1 is closely related to the rotational inertia of the rotor, so the motor's Back Electromotive Force VBEMF1 will change with a much slower pace (actually, the VBEMF1 is nearly unchanged). Under this condition, the time for the upper gate of power switch (i.e. PSW1 in first motor driving state) to stay in the OFF state is much longer than the time to stay in the ON state (in a PWM cycle), and since the motor's Back Electromotive Force VBEMF1 keeps relatively stable, the motor current IL_1 will exhibit stronger downward tendency than upward, and the average level of the motor current IL_1 will keep falling. When the level of the motor current IL_1 falls below 0 Amp and becomes negative, the reverse current takes place. Please refer to FIG. 3B, which illustrates a schematic diagram demonstrating a direction of the reverse current in the DC motor control circuit 10 while operating in the first motor driving state.
The aforementioned reverse current may damage the circuit under some conditions. For example, while there is reverse current existed, and when the PWM signal again switches from the low voltage level (PWM=Low) to the high voltage level (PWM=High) (equivalently speaking, for the first motor driving state, PSW4 is turned off, and PSW1 is turned on), and since the motor current IL_1 needs to keep its continuity all the time, so the motor current IL_1 will keep running in the original direction (flowing from endpoint OUTB1 to endpoint OUTA1). However, since the power switch PSW4 is shut off, so the motor current can only be directed to the power switch PSW1, passing the endpoint VM1, and then directed to the power supply device 100, and this reverse current may seriously damage the power supply device 100. Furthermore, if the system adds a diode D1 for protecting the power supply device 100, then the reverse current (of the motor current IL_1) can only be directed to the capacitor CVM1 after passing the endpoints VM1, but this will escalate the voltage level on the endpoint VM1, and the control circuit may be burned. Please refer to FIG. 3C, which illustrates a schematic diagram demonstrating the condition when the reverse current happens and flows to the capacitor CVM1 in the DC motor control circuit 10 while operating in the first motor driving state. However, in order to prevent any damage to the circuit, the capacitance of the bypass capacitor CVM1 (as well as the size of the capacitor CVM1) needs to be of a larger value (and so the size of the capacitor will become larger); this will increase the component cost and take more space out of the printed circuit board. To explore the origin of the reverse current, it was mainly because there is a Back Electromotive Force VBEMF1 existed in the DC motor. However, the happening of the Back Electromotive Force VBEMF1 is a natural consequence of the running DC motor MOTOR1, and cannot be avoided. Therefore, designer(s) should consider other ways around to prevent the happening of the reverse current.
By the way, U.S. Pat. No. 7,411,367 discloses a DC motor driving method for preventing another kind of reverse current occurring while switching between the first motor driving state and the second motor driving state, and is not designed for preventing the reverse current induced from instantaneous change from the heavy load to the light load within the same motor driving state. U.S. Pat. No. 7,411,367 is inappropriate to resolve the issue made by the Back Electromotive Force inherent in a DC motor as stated above.